1. Field of the Invention
The present invention relates to a method for producing a textured yarn by false-twisting polyester filament yarn. Particularly, it relates to a so-called in-draw system, in which a partially oriented polyester yarn (hereinafter referred to as POY) is simultaneously drawn and false-twisted in the heat-setting zone.
2. Description of the Prior Art
In a conventional false-twist texturing system for polyester filament yarn, a primary heater is utilized for heat-setting a truly twisted portion of the yarn. Most such heaters are of a contact type comprising a heater plate energized by a heating medium such as Dowtherm.RTM. or by a wire embedded therein. The heater plate has a curved surface and groove provided thereon for retaining the yarn on the heater surface and preventing the yarn from leaving the yarn path due to the torque imparted by the twist. As the heat-set conditions for polyester yarn in the conventional false-twist texturing system, a heater temperature less than 250.degree. C. and a processing time more than 0.17 second are set. These conditions are considered common sense for those skilled in the art as described in pages 90 to 94 of "Technical Mannual for Filament Processing" vol I, published on May 15, 1976 by the Textile Machinery Society of Japan.
The conventional texturing system utilizing a contact-type heater suffers from some problems, however. The graph of FIG. 1a shows the crimpability and dyeability of the conventional textured yarn relative to the heater temperature. As clear from the graphs, the crimpability appears to have a peak for a specific heater temperature X, while the dyeability has a valley for another heater temperature Y. Naturally, the differential coefficients of these parameters relative to the heater temperature are almost zero in the vicinity of the peak or valley. Therefore, even if the heater temperature fluctuates somewhat around that point, the value of the parameter remains almost the same. Thus, an even yarn quality can be expected from a process carried out under such a peak or valley temperature. On the other hand, since the differential coefficient of the parameter becomes larger as the temperature is farther from the peak or valley, when the process is carried out under a temperature in a region away from the peak or valley, the parameter tends to vary largely even with subtle temperature fluctuations.
As also apparent from FIG. 1a, the temperature Y under which the dyeability of the yarn becomes minimum is lower than the temperature X for the maximum crimpability. Since yarn processed under such a lower temperature Y is liable to lack bulkiness and fabric obtained therefrom tends to present a poor hand after post heat-treatment such as dyeing or heat-setting, the temperature Y is not preferable as a heater temperature for the texturing process. In practice, therefore, the higher temperature X is utilized. However, under conventional conditions, there is a relatively large difference between the temperatures X and Y. This means the fluctuation of the heater temperature may be reflected in the dyeability of the resultant yarn.
To overcome the above problems, it has been proposed to use a non-touch type heater, instead of contact-type, i.e., a heater through which the yarn can pass without contacting the heater surface. With a non-touch type heater, however, it is difficult to control a balloon of yarn in the heater zone whereupon the heat-setting effect of the yarn becomes irregular. Moreover, in the prior art, more than 0.2 second processing time is thought to be necessary for the yarn in the heater, even in the case of a non-touch type heater. Therefore, the heater temperature must be lower than 350.degree. C. to protect the yarn from heat damage. In such a lower temperature region, however, the crimpability and dyeability of the textured yarn relative to the heater temperature are not improved even by the utilization of a non-touch type heater.